In 3GPP (Third Generation Partnership Project), according to a radio communication system corresponding to LTE (Long Term Evolution), standards of which are being currently designed, a radio base station is connected to MME (Mobile Management Entity) that is an upper node via a backhaul and performs communication with the MME.
In the same manner, according to a radio communication system corresponding to CDMA (Code Division Multiple Access), a radio base station is connected to MSC/VLR (Mobile Switching Center/Visitor Location Register) that is an upper node via a backhaul and performs communication with the MSC/VLR.
A radio communication system corresponding to the LTE and the CDMA uses a technology of CS-Fallback at the time of a voice communication service when the LTE has no VoIP (Voice over IP) function. In the CS-Fallback, a radio terminal connected to a radio base station (an LTE base station) corresponding to the LTE switches a connection to a radio base station (a CDMA base station) corresponding to the CDMA and starts voice communication.
In a radio communication system using the CS-Fallback, a radio terminal always connects to the LTE base station. For this reason, the radio terminal performs the transmission of a call-issuance request or the reception of a call-up at the time of a call arrival via the LTE base station in voice communication. Then, the radio terminal performs handover that switches a connection from the LTE base station to the CDMA base station. After the handover, the radio terminal performs the transmission of a call-issuance request or the reception of a call-up at the time of a call arrival via the CDMA base station, and starts the voice telephone communication.
At the time of a disaster such as an earthquake, it is expected that many persons perform voice communication for the purpose of safety confirmation, a call for help and the like. For this reason, since there is an increase in various processes such as handover in the CS-Fallback, it is expected to reach a state in which congestion easily occurs.
Furthermore, in the radio communication system using the CS-Fallback, the radio terminal needs to be connectable to both the LTE base station and the CDMA base station. However, at the time of a disaster, it is probable that there occurs an obstacle such as a fault of a radio base station or cutting of a backhaul line. Particularly, in the case in which an obstacle occurs in the LTE base station, even when the CDMA base station normally operates, the radio terminal is not able to perform the voice communication.